Single-ended, single-based, pinch-sealed high-pressure discharge lamps of small power rating, as defined, are frequently used for interior illumination, display, and store window illumination and the like, and particularly in situations where the lamps operate continuously for substantial periods of time. The lamps have high light output and good color rendition. Examples of such lamps are described in the referenced U.S. Pat. Nos. 4,717,852, Dobrusskin et al, assigned to the assignee of the present application, and 4,998,036, Matsuura et al.
The lamps typically have a discharge vessel of quartz glass which, frequently, is surrounded by an outer bulb. The discharge vessel has two electrodes located therein between which an arc is struck. The electrodes are angled off to face each other at electrode tips. The discharge vesel or arc vessel has an ionizable fill, usually including an ignition gas, metal vapors and metal halides.
Manufacture of single-ended pinch-sealed lamps, particularly with small discharge vessels, is substantially more difficult than double-ended pinch-sealed lamps. In manufacture of a single-ended lamp, the single pinch seal has a tendency to distort the symmetry of the discharge chamber within the discharge vessel much more than if a double pinch-sealed vessel is used, that is, if the lamp is a double-ended pinch-sealed lamp. The operating pressure of single-ended lamps is usually higher, up to about 50 bar, due to the smaller electrode spacing and the more uniform heating of the discharge chamber. Consequently, the danger of rupture or bursting of the discharge chamber is higher than in a comparable double pinch-sealed lamp, where the pressure rises usually only up to about 28 bar. Very low powered lamps, particularly lamps below 100 W, hardly ever use double pinch-sealed or doubled-ended discharge chambers because heat losses are more pronounced than in single-ended versions. The volumes of the discharge chamber are very small, typically only between about 0.1 to 3 cm.sup.3. These small volumes require meticulous care in the manufacture, in order to reduce the range of variation in the volume of the discharge vessel, and thus to reduce changes in characteristics of the discharge vessel within a given rating to an acceptable limited degree.
The referenced U.S. Pat. No. 5,037,342, Barthelmes et al, describes, as background, a typical sequence of manufacturing steps. A quartz tube is so shaped that a pumping or exhaust tube is formed, and then the tube is heated and, upon introduction of an inert gas under overpressure, the discharge volume is formed by blowing pressurized gas into the soft quartz-glass tube. The blowing region is unsupported, so that the discharge volume is blown without support or back-up. In a subsequent step, and while an inert gas is used to flush the interior of the vessel, the pump tube can be tipped off and the opposite end of the glass tube is pinch-sealed by two pinch-sealing jaws, after the glass has been suitably heated to permit deformation.
It has been found in practice that the free or unrestrained blowing of the glass results in discharge chambers, the volume of which varies considerably between serially manufactured lamps. Further, the subsequent pinch-sealing has the tendency to deform the discharge vessel subsequent to blowing, which additionally contributes to the undesired spread in characteristics of the lamps, and which leads to a non-homogeneous distribution of wall thickness of the discharge vessel.
The referenced U.S. Pat. No. 5,037,342, Barthelmes et al, therefore proposes not to use a pumping tube and, instead of the pumping tube, to close off the end of the glass vessel by a form roller and, by blow-molding, to precisely define the final form of the arc tube or arc vessel to be made. Then, after flushing and introducing the fill through the still open second end of the tube, the glass tube is pinch-sealed.
It has been found that this method, also, can be improved. The fill already has been introduced during heating preparatory to the final pinch-sealing step, which requires cooling of the fill, while the portion of the quartz tube to be pinch-sealed must be heated. This is a complicated, and hence expensive step. Also, the already precisely preformed discharge chamber can be deformed again during the pinch-sealing step.
The referenced German Patent Disclosure Document 39 39 193, Heyde et al, describes a method for low-power halide metal discharge lamps in which, first, a pinch seal is formed and, at the same time, a discharge chamber is preformed. Only thereafter is the discharge chamber blown to receive the finally shape--apparently over the still open stub. This arrangement permits a better control of the volume of the discharge vessel and to maintain the volume almost precisely as desired--at the cost, however, of a time-consuming and energy-intensive manufacturing process since the form-blowing requires a second heating step for the discharge vessel.